Section 4: Highways Downstream of Dams

Peak Discharge

Urban development nearly always increases the runoff rate.
Therefore, affected counties and municipalities often require that
reservoirs be constructed on the primary and secondary drainage channels
to minimize the effect that land development has on the storm runoff
rate. This type of flood control requirement is a popular and permanent
fixture in Texas.

Reservoirs upstream of a highway usually reduce the peak discharge
reaching the highway for a selected frequency of storm runoff.
This reduction is due to flood storage in the reservoir. Documentation
for the design of large reservoirs is ordinarily complete and comprehensive;
smaller reservoirs, however, often are not as well documented.
Therefore, the TxDOT analysis often requires that the floods be
analytically routed through the proposed storage areas to determine whether
or not the required or desired reduction in the peak is accomplished.

Scour Considerations

Reservoirs can contribute to clear water scour downstream
of the discharge point. Significant sediment deposition usually
occurs within the reservoir whenever the reservoir tributary streams
have appreciable sediment loading. As a result, water flowing out
of the reservoir can be deprived of sediment, causing clear water
scour to the banks and around bridge piers.

Design Adequacy

The TxDOT hydraulic designer should confirm with the reservoir
agency that the reservoir has been inspected for structural adequacy
and hydraulic adequacy. Unless the reservoir is consistently maintained
and operated to reduce the flood peak, the reservoir should not
be expected to provide consistent flood attenuation for a downstream
culvert, bridge, or highway and should be ignored.